Abstract

A period of atoll emergence results in the development of a karst surface on the atoll. Upon resubmergence, this surface is preserved as a subsurface solution unconformity that can be identified by petrographic characteristics and a distinctive stable isotope signature imprinted by subaerial or phreatic diagenesis. Determination of the magnitude of Miocene sea-level falls by considering subsidence rates and depths to solution unconformities must take into account the lowering of the emergent atoll surface by dissolution. Previously, the depth to a Miocene solution unconformity below Midway Atoll was assumed to record a maximum highstand of sea level that preceded a period of island emergence. However, because of surface dissolution, the solution unconformity must represent some elevation between a past highstand and a subsequent lowstand. The subsurface stratigraphy of Midway Atoll shows that a late Miocene sea-level fall of 75–125 m below present sea level took place. The magnitude of this fall is greater than that previously determined, on the basis of stable isotope data, from Midway Atoll but approximates Miocene sea-level falls determined by seismic stratigraphic methods.